Research

Antiviral innate immunity mediated by RNase L, a uniquely-regulated nuclease

RNase L is a principal mediator of the innate antiviral response and is thus critically important for human health.  Virus replication in higher vertebrates is restrained by interferons (IFN) that cause cells to transcribe genes encoding antiviral proteins, including the 2'-5' oligoadenylate synthetases (OAS). The viral pathogen associated molecular pattern (PAMP), double-stranded RNA (dsRNA), activates OAS to produce 5'-phosphorylated, 2',5'-linked oligoadenylates (2-5A) whose function is to stimulate RNase L. The OAS-RNase L system thus constitutes a classical innate immune pathway that rapidly responds to PAMPs (here, dsRNA) to produce a broadly active antiviral response.  In our prior studies, we cloned RNase L, knocked it out in mice, established its antiviral and apoptotic activities in vivo, and determined that it initiates transcriptional signaling pathways. In a recent development, we have found that RNase L cleaves self-RNA to generate small RNA products that stimulate IFN-beta synthesis through the RNA recognition receptors, RIG-I and MDA5, and their adapter IPS-1.  In this manner, RNase L acts to perpetuate and amplify innate immunity supporting a broader antiviral state in the organism.  We have also identified small molecule activators of RNase L that possess broad-spectrum antiviral activities. Our future goals are to probe fundamental events and biologic endpoints surrounding RNase L that impact on viral lifecycles.  Our hypothesis is that RNase L inhibits viral replication by cleaving viral and cellular RNA, initiating a stress-response pathway and signaling expression of antiviral genes.

Role of viruses and genes in prostate cancer.

Prostate cancer is the leading cause of non-cutaneous malignancies and the second leading cause of cancer-related deaths among American men.  Hereditary prostate cancer (HPC), which accounts for 43% of early onset cases and about 9% of all cases, is due to germline mutations in HPC genes.  HPC1 encodes RNase L, an essential protein in antiviral innate immunity that has been the focus of our studies for nearly three decades. Genetic evidence that an antiviral gene suppresses prostate cancer led us to examine the possibility that chronic viral infections might predispose men to prostate cancer.  Together with our collaborators, J. DeRisi & D. Ganem (UCSF) and E. Klein (Cleveland Clinic), we discovered a new human retrovirus, xenotropic MLV related virus (XMRV), in tumor-bearing prostate tissues. Remarkably, XMRV was present in prostate tissues of men that are homozygous for a reduced activity variant of RNase L, but rarely in men with wild type RNase L.  In vivo, XMRV was observed to infect prostate stromal cells, in particular cancer associated fibroblasts.  We further showed that XMRV is an infectious virus susceptible to inhibition by interferon (IFN) and its effector, RNase L.  Our studies with collaborators (S. Chow, UCLA) mapped XMRV integrations sites in human tumor DNA and we identified the cell surface receptor for XMRV as the xenotropic and polytropic receptor 1 (XPR1).  Our hypothesis is that RNase L suppresses prostate cancer in part by restricting XMRV and other viral infections.  Our goals in these studies are to probe the fundamental roles of RNase L and XMRV in tumor biology and to explore their clinical significance.